This text discusses a few technical aspects of the shutterless mode of EIT.
My discussion is based on the shutterless mode sequence of May 13, 1998.
Specifically, I have a look at instrumenal effects such as the 'smearing' of the
image due to reading out the CCD with the shutter open and skipping ccd clears.
To conclude I give a few recommendations for new shutterless mode runs by EIT
and for future EIT-like instruments.

The May 13, 1998 shutterless mode sequence (efz19980513.173209) had the
following setup (see Fig. 1):

a 128x96 subfield in the SW quadrant was choosen ( [512:639,352:447])

read-out port B (bottom left)

block east filter

no ccd clears during the whole sequence to keep maximal cadence

230 images between 17:32 and 18:29, average cadence 14.9 s

For the following discussion we also note the following images on the same day:

filename

image type

filter

efz19980513.165311

FOV test image

Al sup

efz19980513.170145

195 fdfr

Al +1'

efz19980513.183451

195 fdfr

Al +1'

General procedure

After being exposed, a shutterless mode image of the subfield 'FOV' is
transferred to the readout port. According to Delaboudiniere et al
(SolPhys 1995, 162, p. 304) reading out a subfield takes the following three
steps:

dumping of all lines ahead of the fov. The lines preceeding the subfield
are discarded at a rate of 0.480 ms per line. After this 'edge 1' has
shifted to the left edge of the CCD and the charges
collected in region FOV move to region A (see Fig. 1). Since the shutter
remains open, the FOV image is contaminated by photons collected during
the transfer through the non-occulted part of region B and through the
remaining transfer path in the FOV itself.

actual read out of the fov image now present in region A at a rate of
20.8 ms per line. Note that

thanks to the occulting mask the fov image is no further contaminated.

every time a column is read out, a 'black' column appears at the right
hand side of the CCD (region D) which then further drifts leftwards when
new lines are read out. In this process a smeared out image of the
far western side of the sun is collected in region D.

dumping of the remainig 1024-512-128=384 lines following the choosen
subfield, again at a rate of 0.480 ms per line.
Note that in this process the smear out image that was collected in
region D is moved in the FOV image. Moreover
this smear out image of the far western side of the sun
is contaminated during the transfers through region 'C' and through
the remaining transfer path in the FOV itself.

It is important to note that, since we do not do any 'ccd clears',
this contaminated smear out image remains in the fov when the exposure
of the next image starts.

Figure 1.

Estimation of the contamination

So, to summarise a subfield 'image' consists of the following contributions:

smear out of region D with an effective exposure time of
t1 = 128 x 20.8 ms = 2.6624 s

smear out of the transfer through region C and the FOV with an effective
exposure time of t2=384 x 0.480 ms =0.18432 s

true solar image in the FOV region with an at this stage unkown exposure
time of t3 s.

smear out of the transfer through the FOV and the unocculted part of
region B. The effective exposure equals the transfer time from the RHS
of the FOV (512+128) to the RHS of the occulting mask (guess: 341) which
gives: t4= (512+128-341)*0.480 ms = 0.14352 s

Finally, these 4 contributions are being transferred another 341-128=213
lines (which takes 213*0.480 ms = 0.10224 s) and are read out in 2.6624 s.
So they spend T_o= 2.76464 s under the occulting mask.
This value for T_o can be confirmed by noting (see Fig.2) that the average
intensity of a shutterless mode image scales as the
the time interval between the observation of that image
and the preceding image plus an offset:

average_intensity(i) ~ obstime(i)-obstime(i-1) - ( 2.71 +/- 0.05 s)

Since we do not do any ccd clear the total exposure (t1+t2+t3+t4) time
together with the time T_o spent under the occulting mask must equal
the time interval between the observation of the preceeding image
and the present image:

obstime(i)-obstime(i-1) = t1+t2+t3+t4 + T_o

This allows us to derive the effective exposure time t3 of the true solar
image in the fov:

Let us now, starting from the nearby full disc image efz19980513.183451
try to estimate what the combination of contaminations 1,2 and 4 look like.
Taking the appropiate integrations from this full disc image and using
the above deduced 'effective exposures' for each contamination, we sum
up the

contaminations 2 and 4 into a charge transfer contamination template
with the following statistics in DN:
Minimum = 3.9, Maximum = 9.1, Average = 6.4+/-1.3

contamination 1, due to the read-out process of the previous image gives
a read-out template with the following statistics in DN:
Minimum = 0., Maximum = 52.3, Average = 13.0+/-11.0

Summing these two contributions gives the total contamination template which
has the following statistics expressed in DN:
Minimum = 4.5, Maximum = 58.8, Average = 19.4+/- 11.0

Verification of the deduced total contamination template

Figure 3.

Let us now try to find observational evidence of the above scenario.
Fig 3A shows a subfield extraction of the fdfr image efz19980513.170145 and
Fig 3B shows the subfield test image efz19980513.165311. Both are taken
with the normal operational mode of shutter opening and closing.
Their difference, shown in Fig. 3c, are due to

active region pattern due to solar variability between 16:53 and 17:01

vertical stripes due to the difference between the BLOCK_EAST filter and the Al + 1 filter

It's important to note that the difference in filter only produces vertical
stripes, indicating that the two corresponding gridpatterns are well aligned
in the y-direction but somewhat offset in the x-direction. A fortunate
consequence is that although efz19980513.18345 has a different grid pattern
than the shutterless mode sequence, we can still use it to estimate the smear
out contamination since horizontal differences are absent and vertical ones
get smoothed anyway when smearing.

In the next line, Fig. 3d shows a subfield extraction of the fdfr image
efz19980513.183451 (Al + 1) taken with the normal operational mode of shutter
opening and closing, while Fig. 3e shows the last but one image in the
shutterless mode sequence (BLOCK_EAST). Their difference (Fig. 3f) shows all
features allready seen in Fig 2c, but due to shutterless mode operation we
now have an additional left-bright right-dark trend.
We have enhanced this trend by applying horizontal smoothing to
remove the grid modulation and median filtering to reduce the noise.
This gives Fig. 4a which can be compared to the derived contamination template
Fig. 4B. (shown in the same grey scaling). The bright white regions in Fig. 4a
are regions where the shutterless mode image has a lower value than the
fdfr subfield image, due to intrinsic solar changes.

On a global scale, Fig. 4a and Fig 4b show the same left to right dimming
pattern. This becomes especially clear when plotting in Fig. 4c
the vertical average of both Fig. 4a (solid line) and Fig. 4b (dotdashed line).
Both curves have the same amplitude and the same trend. The largest
differences (approx 5 DN, see Fig. 4d) are seen in between column 50 and 100,
which is probably due to solar variability.
Also several of the horizontal stripes in the derived contamination
(Fig. 4b) are recognisable in Fig. 4a: in Fig. 4e we a vertical cut
through Fig. 4a (solid line) and Fig. 4b (dotdashed line) averaged over
the first 10 columns at the left. The profile of both curves is clearly
similar, though the derived contamination apparently underestimates
the observed difference by roughly 5 DN. Nevertheless this difference can
be compensated by assuming that the exposure time mentioned in the
fdfr image header was slightly too high (only 1.5 %).

By comparing the contamination template calculated from the
18:34 fdfr image with templates based on the 13:14, 17:01 and 19:13
fdfr image we conclude that the temporal variations of the total
contamination template are typically
less than 10% on the timescale of 1 hour which in absolute numbers means an
uncertainty of only 2 DN.

Figure 4.

Conclusion and Recommendations

With the above analysis we have shown that due to the shutterless mode
operation in combination with the absence of any ccd clears during the
sequence:

the images are contaminated by charges collected during the charge
transfer phase and during the reading out of the preceeding image

both contaminations add up to around 10 % of the DN-values in the
darkest regions of the images and much less in the brighter ones

by intrgrating along the transfer/read out paths in a nearby fdfr image,
these contaminations can be relatively well estimated and can thus be
subtracted from the shutterless mode images.

after doing this the residual contamination is estimated to be
less than 2 %.

Since this contamination can be removed relatively well, it is shown
that skipping all CCD clears is an allowable option to enhance the
observation cadence. For future shutterless mode
runs on EIT, we recommend to add full disc images as close as possible
before and after the run, both in the Al +1 filter as in the blockeast
(or west) filter. This will allow an even better estimate of the contamination
template and its evolution during the sequence. In case no ccd clear are
programmed during a shutterless mode run, the occulting filter (block east
or block west) should be choosen, not according to the selected read out port,
but depending on the relative brightness of the far west or the far east
of the full disc image.

For future EIT like instruments we propose to consider the implementation
of 'partial ccd clears' which would allow to fine-tune the removal of
contamination from the fov (and only there) without degrading excessively the
observation cadence. In case of demanding mass/budget restrictions one could
also consider to remove the occulting half masks completely, since our analysis
shows that the resulting smear-out contamination can be well estimated and
removed to within a few percent.